From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A27A4199931 for ; Sun, 17 Aug 2025 06:00:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755410413; cv=none; b=GFmcGEPM1KkuR4wnb8wkQ5+YGJ6O9gh29g7+sQZhwSOlFLe8LD2X9VrsNbpmycwcpZQBF4+QcrmVt0mh8WcvhPYBrVAHHwiZ6OqJFHwyvtCHr7Gjq+xJJ1Mj/vVm11iBWui5C5CT06B3FbKATUIR/ildx0DL907XLMON7MVvEDg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755410413; c=relaxed/simple; bh=EZ/OY9/PmCsYJzwheJnYYUJ7szKgYoULjAW1iX8SyHo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=bujgOOX3w+gbEkOzNzRuoXRde++0CLqkk2YUDaYIfgnHen0q5acMmXKcV2ulPEcW5FYLj0hRtrwgIaWm0ljq5yggqgH4ovOIq+eSOzU8a+DYZM6iKboJUY6EI7wV1Zl8jlircpo1cZuISqSivC6H+tNYVLLJcMrNBfm7Uu0ZQ3M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=Yq/Sj56+; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="Yq/Sj56+" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C4C04C4CEEB; Sun, 17 Aug 2025 06:00:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1755410413; bh=EZ/OY9/PmCsYJzwheJnYYUJ7szKgYoULjAW1iX8SyHo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Yq/Sj56+aodXKdVkbmPoMpTrjRhrWbGlahXTvrzo4v1d7UnD/zr8Cu/XVi3/Jo3D1 pJUZVPTSUqL7H1pnPQl/CYQhMRbxjmkQPto1I5KgFn6oktQREbUPqj8WNhZDjOs8Lz nVC83zwDm9ZqGiWdocS8XAFt714KgBLXssqZkcT4= Date: Sun, 17 Aug 2025 08:00:09 +0200 From: Greg KH To: Benno Lossin Cc: Simona Vetter , Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , =?iso-8859-1?Q?Bj=F6rn?= Roy Baron , Andreas Hindborg , Alice Ryhl , Trevor Gross , Danilo Krummrich , rust-for-linux@vger.kernel.org Subject: Re: [PATCH v4 0/4] Untrusted Data API Message-ID: <2025081746-aspirate-diminish-29cd@gregkh> References: <20250814124424.516191-1-lossin@kernel.org> <2025081416-sufferer-economist-3f00@gregkh> <2025081448-creation-timid-b972@gregkh> <2025081435-broker-valium-7b22@gregkh> <2025081505-facial-cyclic-af25@gregkh> Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Sat, Aug 16, 2025 at 12:22:04PM +0200, Benno Lossin wrote: > On Fri Aug 15, 2025 at 4:19 PM CEST, Greg KH wrote: > > On Fri, Aug 15, 2025 at 09:28:59AM +0200, Benno Lossin wrote: > >> On Thu Aug 14, 2025 at 8:26 PM CEST, Greg KH wrote: > >> > On Thu, Aug 14, 2025 at 07:23:45PM +0200, Benno Lossin wrote: > >> >> On Thu Aug 14, 2025 at 5:42 PM CEST, Greg KH wrote: > >> >> > On Thu, Aug 14, 2025 at 05:22:57PM +0200, Benno Lossin wrote: > >> >> >> On Thu Aug 14, 2025 at 4:37 PM CEST, Greg KH wrote: > >> >> >> > On Thu, Aug 14, 2025 at 02:44:12PM +0200, Benno Lossin wrote: > >> >> >> >> I didn't have too much time to spend on this API, so this is mostly a > >> >> >> >> resend of v3. There are some changes in the last commit, updating to the > >> >> >> >> latest version of Alice's iov_iter patche series [1] & rebasing on top > >> >> >> >> of v6.17-rc1. > >> >> >> >> > >> >> >> >> I think we should just merge the first two patches this cycle in order > >> >> >> >> to get the initial, bare-bones API into the kernel and have people > >> >> >> >> experiment with it. The validation logic in the third patch still needs > >> >> >> >> some work and I'd need to find some time to work on that (no idea when I > >> >> >> >> find it though). > >> >> >> > > >> >> >> > Nice, thanks for reviving this! > >> >> >> > > >> >> >> > And we should at least add an example using it, otherwise it's not going > >> >> >> > to help out much here. Add it to the misc device driver api? > >> >> >> > >> >> >> You mean `rust/kernel/miscdevice.rs`? What parts of that API are > >> >> >> untrusted? > >> >> > > >> >> > mmap() is, but you can't do anything about that... > >> >> > >> >> Which parameter is untrusted there and why can't I do anything about it? > >> > > >> > The whole memory range is untrusted as to what is written there, sorry, > >> > it was a bad attempt at a joke, the kernel never gets a chance to know > >> > what is happening. > >> > >> Ahh that flew over my head :) So we'd either build `Untrusted` directly > >> into the `VmaNew`/`VmaRef` abstractions -- or if there is a way to have > >> a trusted `VmaNew`, we can wrap it in `mmap`. > > > > Nah, I wouldn't worry about that, mmap() doesn't seem to cause security > > issues as by definition it is just allowing userspace to read/write > > anything to that device or memory, and the kernel doesn't even see it. > > Because of that, the kernel can't really be "broken" with invalid data > > there (hardware can, of course, but that's userspace's fault the kernel > > is just setting up a pipe here.) > > I'm guessing that the kernel doesn't usually read these? (if we don't > have a way to read them from the Rust side, we don't need `Untrusted`) Correct. > >> >> If so we probably > >> >> should have a single parameter so users can verify them at the same > >> >> time. Or am I thinking of the wrong thing to verify? (`file` should be > >> >> already in kernel memory, right?) > >> > > >> > Both are usually verified at different places, first `cmd` tells what > >> > `arg` is going to be, and then the code goes off and parses whatever > >> > `arg` points to (or contains for simple ioctls). > >> > > >> > And for some, `arg` is just a place to write something back, so `arg` > >> > needs no verification for them, it depends on what `cmd` is. > >> > >> I still think grouping them together makes sense, since in the > >> validation function you'd want access to both, right? And these use > >> cases seem perfect for enums: > >> > >> enum MyIoctlArgs { > >> WriteFoo(UserPtr), > >> VerifyBar(UserPtr), > >> // ... > >> } > >> > >> And then in the validation function you can do: > >> > >> const WRITE_FOO_CMD: u32 = ...; > >> > >> fn validate(cmd: u32, arg: usize) -> Result { > >> Ok(match cmd { > >> WRITE_FOO_CMD => MyIoctlArgs::WriteFoo(UserPtr::from_addr(arg)), > >> VERIFY_BAR_CMD => MyIoctlArgs::VerifyBar(UserPtr::from_addr(arg)), > >> _ => return Err(EINVAL), > >> }) > >> } > >> > >> So when wrapping them together we could have: > >> > >> pub struct IoctlArgs { > >> pub cmd: u32, > >> pub arg: usize, > >> } > >> > >> And then change the `MiscDevice::ioctl` function to: > >> > >> fn ioctl( > >> _device: ::Borrowed<'_>, > >> _file: &File, > >> _args: Untrusted, > >> ) -> Result > > > > I think the problem with this is you now end up with the "I verified > > this is ok, so now I will copy it" bug, where userspace will race with > > the kernel and modify the data after verification but before copying. > > > > Note, Windows is full of these types of bugs as they don't do a call to > > copy_from_user(), but usually just poke at the data directly. Linux at > > least forces a copy_from_user() call, but it doesn't always work in that > > you still have to validate the data is correct and people forget. > > > > So as long as we can copy the data from userspace first, and then > > validate it, and keep that validated copy around to use, that's great. > > I can't really determine above if that is the case or not, sorry. > > So if I understand the current API correctly, the `arg` and `cmd` > parameters are copied from userspace (or rather they are direct > parameters of the syscall), so you can't have the copy-after-validation > bug there. Then in the `ioctl` function one currently just looks at > `cmd` & then decides what to do with `arg`, possibly doing a > `copy_from_user`. > > In my suggestion for the API, we just change the first part of the > current approach, combining the two parameters to `ioctl` into a single > struct & making people parse it using the untrusted API. > > The API that protects you from the copy-after-validation bug is the > `UserPtr` abstraction. And there I also will add the untrusted API. > For example the `UserPtr::read_all` function would become: > > pub fn read_all(self, buf: &mut Untrusted>, flags: Flags) -> Result; > > This means that you can only read the data into an `Untrusted>` > which ensures that the data is validated before use. This API also makes > it harder to have the copy-after-validation bug, since you have to > explicitly call `clone_reader`. Ok, that sounds good!